1. Field of the Invention
This invention relates to a pressurized water reactor system of an advanced design incorporating two-piece drive rods assemblies having selectively and remotely actuable, quick-disconnect couplings, and to methods of performing assembly/disassembly and related maintenance operations on such a reactor.
2. State of the Relevant Art
As is well known in the art, conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials known as poisons, which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor.
Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted to a common, respectively associated spider. Each spider, in turn, is connected by corresponding drive rods to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster.
In certain advanced designs of such pressurized water reactors, there are employed both reactor control rod clusters (RCC) and water displacer rod clusters (WDRC). In one such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters being mounted to a respectively corresponding spider. In the exemplary such advanced design pressurized water reactor, there are provided, at successively higher, axially aligned elevations within the reactor pressure vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome, or head. The lower barrel assembly may be conventional, having mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. Within the inner barrel assembly there is provided a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies.
One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower, overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even through the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations.
The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. Within the calandria and extending between the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, respectively aligned with the rod guides. Flow holes are provided in remaining portions of the calandria plates, intermediates the calandria tubes, through which passes the reactor core outlet flow as it exits from its upward passage through the inner barrel assembly. The core outlet flow, or a major portion thereof, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria.
In similar, parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the head, and which in turn are connected to corresponding head extensions which pass through the structural wall of the head and carry, on their free ends at the exterior of and vertically above the head, corresponding adjustment mechanisms, as above noted. The adjustment mechanisms have correspondiing control shafts, or drive rods, which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders mounting the clusters of RCC rods and WDRC rods, and serve to adjust their elevational positions within the inner barrel assembly and, corrspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core.
In the exemplary, advanced design pressurized water reactor, over 2,800 rods are mounted in 185 clusters, the latter being received within the corresponding 185 rod guides. Of these clusters, 88 are of the WDRC type, divided into 22 groups of four clusters each, the clusters of each group being chosen such that withdrawal of an individual group, or multiple such groups, maintains a symmetrical power distribution within the reactor core. Since each WDRC is approximately 700 lbs. to 800 lbs. in weight, each group of four (4) such clusters presents a combined weight of in the range of from 2,800 lbs. to 3,200 lbs., requiring that a drive mechanism and associated connecting structure for each group of four clusters have substantial strength and durability, and afford a substantial driving force.
Due to the packing density, or close spacing, of the rod clusters and their associated guides, severe spacing requirements are imposed, both within the vessel and with respect to the rod drive mechanisms, including both the water displacer rod drive mechanisms (DRDM's) and the control rod drive mechanism (CRDM's). The critical spacing requirements were not experienced in reactors of prior, conventional types, which did not employ WDRC's and correspondingly did not employ DRDM's. In reactors of such conventional designs, ample spacing was available above the dome, or head, of the vessel for accommodating the required number of mechanisms for driving the RCC's. Particularly, the CRDM's of well known, electromechanical type associated with corresponding clusters of RCC's, were mounted in generally parallel axial relationship, vertically above the dome or head of the vessel and extended in sealed relationship through the head for connection by suitable drive rods to the associated RCC's, and provided for selectively controlled gradual raising and lowering of the RCC's for moderating the reactor energy level, or for rapidly lowering same in the case of shutdown requirements.
In reactor systems of the advanced design herein contemplated, whereas the same mechanisms conventionally employed for the CRDM's functionally are acceptable for adjusting the WDRC's, due to the increased number of rod clusters (i.e., the total of RCC's and WDRC's s), the conventional CRDM's are unacceptable mechanically since they are too large. Various alternative mechanisms have been studied in view of this problem. For example, roller nut-drives were considered, but were determined to produce insufficient lifting force. Accordingly, a substitute DRDM has been developed which utilizes a hydraulically operated piston which is attached through a corresponding drive rod to each group of associated WDRC's, and which mechanism satisfies the spacing limitations, permitting mounting thereof above the head or dome of the vessel in conjunction with the conventional CRDM's. An example of such a hydraulically operated drive mechanism for a WDRC is shown in U.S. Pat. No. 4,439,054--Veronesi, issued Mar. 27, 1984 and assigned to the common assignee hereof.
A further critical design criterion of such reactors is to minimize vibration of the reactor internals structures, as may be induced by the core outlet flow as it passes through the reactor internal structures. A significant factor for achieving that criterion is to maintain the core outlet flow in an axial direction throughout the inner barrel assembly and thus in parallel axial relationship relatively to the rod clusters and associated rod guides. This is achieved, in part, by the location of the water inlet and outlet nozzles at an elevation corresponding approximately to that of the calandria assembly, and thus above the inner barrel assembly which housed the rod guides and associated rod clusters, as above noted.
The configuration of pressurized water reactors of the above described, advanced design has resulted in the requirement of increasing the axial height, or length, of the reactor pressure vessel, compared to that of conventional such reactors. The increased vessel height concomitantly imposes the requirement that the drive rods be on the order of 34' in length, compared to the 24' typical length of drive rods employed in conventional reactor vessels. This increase in the drive rod length poses many problems. At the outset, producing drive rods in a length of 34' or more is extremely expensive, and structures of this size introduce not only special manufacturing problems but as well problems of handling and shipping, including significant expense therefor, compounding the total cost factor. Further, whereas a one-piece drive rod could be produced by welding together, or using a mechanical joint to connect, plural, shorter components, the material employed in the drive rods, e.g., type 403 stainless steel, imposes costly preheat and postheat operations for welding, whereas mechanical joints require expensive buttering operations to permit tack welding of locking pins thereto for completing the mechanical assemblage. Performing either welding or mechanical assemblage operations in the field is impractical, and forming one-piece drive rods in this manner at a factory for subsequent transportation to the field site imposes the same problems as before noted.
Aside from the basic problems of fabrication, handling and transportation of one-piece drive rods sufficient in length to satisfy the requirements of pressurized water reactors of the advanced design, numerous other problems are presented thereby. Mechanically, such one-piece drive rods are difficult to manipulate, both in the assembly of a reactor incorporating same and in performing periodic maintenance operations. For example, alignment and assembly of the drive rods with the corresponding rod clusters is made extremely difficult by the increased length, introducing risks of damaging interior structures, such as the rod guides within the upper barrel assembly. At a minimum, structural damage by way of dents or other deformation of the rod guides cannot be tolerated in view of the potentially critical adverse flow characteristics and related vibration conditions which might result. Any such damage, of course, ultimately presents a problem of potential failure, over time. Further, distortions of rod guides may impeded the required, free axial movement of the rod guides of the associated clusters which, at a minimum level, would increase the rate of wear and, at a more extreme level, could prevent the smooth, axial/transational movement of the rod guides and associated clusters as is required for their intended control purposes, as above discussed.
In addition to the mechanically related problems and high cost factors created by and related to the excessively long drive rod structures required in such advanced design, pressurized water reactors as here considered, other critical problems as well are presented. For example, to perform routine maintenance operations, whether for inspection and/or replacement of parts, including rearrangement and/or replacement of partially used or spent fuel rod assemblies, the vessel must be disassembled, or dismantled. Typically, the head assembly first is removed and the drive rods removed, following which the various internals of the vessel are withdrawn, usually in successive stages or operations, thereby to gain access to the rod clusters and finally to the fuel rod assemblies. In some systems, the rod clusters must remain inserted into the fuel rod assemblies during refueling operations.
Due to the required length of the drive rods in the advanced design reactor vessels, these required maintenance functions present additional problems if drive rods of the required length of conventional one-piece construction were employed, and conventional methods were followed. Since the drive rods are immersed in the reactor coolant fluid, they are exposed to radioactivity within the vessel. Thus, after being withdrawn to permit disassembly of the vessel, and particularly in subsequent, reassembly operations, the rods would be exposed to the atmosphere above the boron-enriched water and thus prevent the serious and unacceptable risks of exposing maintenance personnel to undesired levels of radioactivity. The alternative of enlarging the containment structures to permit increasing the level of boron-enriched water, such that the excessively long drive rods remain immersed at all times, imposes an unacceptable increase in cost of the containment structure and of maintenance operations, in view of the required substantial increase in volume of boron-enriched water necessary for filling same to the required level to maintain the longer drive rods submerged. Even if the cost factors relevant to providing a sufficient depth of boron-enriched water were acceptable, the increased depth would present concomitantly increased difficulties in the reassembly operations, because the exceedingly long drive rods would have to be controlled in alignment though even a greater depth of water for reconnection, or recoupling, to the respectively corresponding rod clusters.
Accordingly, the increased vertical height, or longitudinal size, of the pressure vessels reuired by pressurized water reactors of the advanced design herein contemplated introduce problems, particularly relating to the drive rod structures, not encountered heretofor with reactor systems of conventional designs. Accordingly, there are no known solutions available, arising out of the prior art of conventional style reactors, for solving the complex problems addressed by the present invention.